The fast mechanical response of the mammalian auditory outer hair cell is believed to be the cellular basis of the positive feedback mechanism required for the fine tuning process of the hearing organ. We showed that the fast mechanical response of the cell is membrane potential dependent. We found that a simplified membrane model can describe the elastic property of the cell and were able to evaluate the force produced by the cell for the first time. It turned out that the efficiency is 0.5 nN/Mv for a single cell, and this indicates that the mechanical feedback resulting this effect is of the same order of magnitude as the mechanical signal on the cellular level. In the process of investigating what kind of physiological signals cause athe fast response, we demonstrated that the lateral wall can also be a mechano-receptor in addition to the stereocilia. We observed macroscopic current produced by osmotic stress, attributable to stretch- activated potassium channels. The endocochlear potential, which is the main energy source of the mechano- electrical transduction in the inner ear, is attributed to the marginal cell of stria vascularis. With the whole-cell recording method, we found that an inward current of the marginal cell is blocked by micromolar concentration of amiloride. This is suggestive that the cell has amiloride-sensitive sodium channel similar to other epithelium cells such as the ones in the kidney. This current can, at least partially, account for the unusually positive membrane potential of the marginal cell, and thereby the endocochlear potential.